afaik, NFS is the supported solution and the only solution since vCloud Director released.
I'm not sure exactly how well clustering would work with a software iSCSI target, as it would be formatted with EXT3/EXT4 format (likely). This is because iSCSI would be block level storage, and each system might allocate and overwrite each other's actions.
Say, in the case of ESX, we format iSCSI in our VMFS format ... which is specifically designed to allow clustering of multiple client systems to the iSCSI LUN/Target (i forget the appropriate term).
I just don't know if ext3/ext4 of similar would work this way, if at all.